A continuous distribution of sound energy, referred to as an “acoustic field”, may be used for a range of applications including parametric audio and the levitation of objects.
By defining one or more control points in space, the acoustic field can be controlled. Each point may be assigned a value equating to a desired amplitude at the control point. An acoustic field exhibiting the desired amplitude at the control points is created by actuating the set of transducers as a phased array. Focusing the energy in the desired control point location implies the transducers are excited at different times such that the waves output from each arrive together.
Specifically, mid-air haptic feedback is generated with array(s) of ultrasonic transducers. Typical arrays contain 256 (or more) transducers, all driven independently, to generate ‘focal points’ through constructive and destructive interference of the audio output from each transducer. To simplify implementation of the signal processing used to stimulate the ultrasonic transducers the fixed frequency digital inputs (often 40 kHz) are typically divided into a fixed number of phases, with each transducer switching synchronously on a phase transition.
Ultrasonic transducers are typically driven with a digital electrical input signal, generating an analog audio output signal. The digital signal is typically originally generated at 3V (or less) by digital signal processing circuits but the ultrasonic transducer must be driven with a higher voltage, typically 20V, to maximize the audio power emitted by the transducer. When the digital control signal switches to 3V the transducer will be driven to 20V. When the digital control signal switches to 0V the transducer will also be driven to 0V. The ultrasonic transducers are typically driven at a fixed frequency (often 40 kHz) corresponding to the resonant frequency of the transducer.
Existing driver circuits can provide direct voltage drive to the transducer, driving the capacitive load to the 20V output voltage through a small (ideally zero) output impedance. Alternatively they can provide current drive, driving the capacitive load to the 20V output voltage by sourcing a significant (typically 100's of mA) current.
The digital control signal is typically ‘level shifted’ from the 3V low voltage to the 20V high voltage with the use of a level shifting circuit. Since the transducer has a significant capacitance, typically 2 nF (or more) then the level shifting circuit must be capable of sourcing (sinking) a significant current for a very short time when the transducer switches between 0V and 20V.
Typical peak switching current for a single transducer with input capacitance of 2 nF switching between 0V and 20V in 100 ns would be given by
  i  =            C      ⁢                          ⁢              dV        dt              =                  2        ⁢                                  ⁢        n        ⁢                                  ⁢        F        *                              20            ⁢                                                  ⁢            V                                100            ⁢                                                  ⁢            ns                              =                        2          *                      1            5                          =                              0.4            ⁢                                                  ⁢            A                    =                      400            ⁢                                                  ⁢            mA                              Local ‘decoupling’ capacitance is typically used to reduce the demand for current from a power supply during this switching, often reducing the peak current to about 200 mA over 200 ns.
With an array of (typically) 256 transducers it will be common for several transducers to switch (between 0V and 20V) at the same time. For example, if 8 transducers switch simultaneously then the peak current required from the power supply would be 8*400 mA or 3.2 A without local decoupling capacitance and 1.6 A with local decoupling capacitance, corresponding to eight driver circuits each requiring peak current of 400 mA.
The peak current in this example would last for 200 ns but if the 40 kHz digital input frequency was divided in (say) 64 phases then each phase would last for 390 ns and the power supply would need to source an average of approximately 0.5 A, with the peak being 1.6 A.
Switching the load on a power supply between 0 A and >1 A every 200 ns can cause major variation on the output voltage of the power supply based on the load transient response. Depending on the power supply characteristics this could cause more than 10% variation (for example) in the nominal 20V output voltage so that actual power supply output voltage is somewhere between 18V and 22V, leading to both an uncontrolled reduction and increase in output audio power and cross modulation between transducers impacting the control point(s).
The example described above requires an average of 0.5 A with a peak of 1.6 A for 8 transducers switching at the same time. If 16 or 32 transducers were to switch at the same time the average (peak) current would become 1 A (3.2 A) and 2 A (6.4 A) respectively.
Typical power supplies have a peak output current they can support. Whatever the peak output current for the power supply, with an array of ultrasonic transducers driven as described there will be an upper limit on the number of transducers that may switch at the same time.
Accordingly, there is a need for an improved transducer circuit that can address the foregoing limitations and drive the transducers in a more efficient manner.